Applications sometimes need to establish and manage a session between computing devices. A session is a set of interactions between computing devices that occurs over a period of time. As an example, real-time communications applications such as MICROSOFT MESSENGER or Voice over Internet Protocol (“VoIP”) establish sessions between communicating devices on behalf of a user. These applications may use various mechanisms to establish sessions, such as a “Session Initiation Protocol” (“SIP”). SIP is an application-layer control protocol that devices can use to discover one another and to establish, modify, and terminate sessions between devices. SIP is an Internet proposed standard. Its specification, “RFC 3261,” is available at <http://www.ietf.org/rfc/rfc3261.txt>. A specification for extensions to SIP relating to event notifications, “RFC 3265,” is available at <http://www.ietf.org/rfc/rfc3265.txt>. Both of these specifications are incorporated herein in their entirety by reference.
Applications may use SIP with another protocol to send or receive information. As an example, an application may use SIP with Real-time Transport Protocol (“RTP”) for transporting real-time data during a session. By using SIP with other protocols, applications can create and manage a session and exchange information during the session. The protocol used with SIP to exchange information may segment the information into messages. Devices may also exchange SIP messages. This exchange of messages during a session is referred to as a “dialog.” SIP may use lower-level communications layers to transport a dialog's messages, such as Transmission Control Protocol/Internet Protocol (“TCP/IP”), which are commonly employed transport- and network-layer protocols.
A SIP network comprises entities that can participate in a dialog as a client, server, or both. SIP supports four types of entities: user agent, proxy server, redirect server, and registrar. User agents initiate and terminate sessions by exchanging messages with other SIP entities. A user agent can be a user agent client, which is generally a device that initiates SIP requests, or a user agent server, which is a device that generally receives SIP requests and responds to such requests. As examples, “IP-telephones,” personal digital assistants, and any other type of computing device may be user agents. A device can be a user agent client in one dialog and a user agent server in another, or may change roles during the dialog. A proxy server is an entity that acts as a server to clients and a client to servers. In so doing, proxy servers intercept, interpret, or forward messages between clients and servers. A redirect server accepts a SIP request and generates a response directing the client that sent the request to contact an alternate network resource. A registrar is a server that accepts registration information from SIP clients and informs a location service of the received registration information.
SIP supports two message types: requests, which are sent from a client to a server, and responses, which are sent from a server to a client, generally when responding to a request. A SIP message is comprised of three parts. The first part of a SIP message is a “start line,” which includes fields to indicate a message type and a protocol version. The second part of a SIP message comprises header fields whose values are represented as name-value pairs. The third part of a SIP message is the message's body, which is used to describe the session to be initiated or contain data that relates to the session. Message bodies may appear in requests or responses.
SIP messages are routed based on the contents of their header fields. To be valid, a SIP request should contain at least the following six header fields: To, From, CSeq, Call-ID, Max-Forwards, and Via. The To header field indicates the logical identity of the recipient of the request. The From header field indicates the logical identity of the initiator of the request. The Max-Forwards header field indicates the number of hops a request can make before arriving at its destination. As an example, if a message from device A transits device B before arriving at destination device C, the message is said to have made two hops (e.g., devices B and C). The Via header field indicates the path taken by the request so far (e.g., a sequence of network addresses of devices through which the request has transited) and indicates the path that should be followed when routing the response. A header may also contain Record and Record-Route fields that are used to indicate that future requests and responses should be routed through an indicated device. Various network devices may insert Record-Route header fields when forwarding a SIP message in an attempt to force subsequent messages in a dialog to be routed through the device. The Record-Route header field may contain an identifier (e.g., network address) for the device and parameters. Devices that handle a message may force the message to be routed to devices listed in a message's Route header field. The Route header field values may be based on the Record-Route header field values inserted by devices. These and other header fields are described in the SIP specification referenced above.
A SIP server may be a single point of failure and may be unable to support large numbers of simultaneous sessions. The server could be a single point of failure because all SIP messages from or to a client may transit the server and, if the server fails, the client could be unable to participate in SIP dialogs because a location service may associate the client with the failed server. A single server may be unable to support large numbers of clients because of hardware (e.g., processor or memory) and network bandwidth limitations. In such cases, large numbers of clients can be denied server support, causing failure of the applications being performed on these clients.
An effective approach to overcome these disadvantages of SIP would have significant utility.